Good point re. power/energy, although over long races (i used mine for 12 hour off roads, it allowed me to finish (some) races due to reduction in fatigue as opposed to being a broken heap & not finishing.
A suspension post would definitely sap a bit of power but unless you’re going for a top ten , in which case you’re double hard anyway!, there’s not a huge amount of difference.
Looking at how a parallelogram post and a telescopic seatpost works could also prove different energy v power results as well as the comparable weights. ;-)